Gleason : Unsolved problems of the prairies 267 



prairies in the Chicago area, described by Cowles,* and the sand 



r 



prairies along the Illinois River,f and in other parts of north- 

 western Illinois. J 



4. Comparisons with the still existing prairies farther west, 

 as described by ecologists from Iowa, South Dakota, § and 

 Nebraska^ 



By carefully combining observations taken from as many 

 standpoints as possible we should still be able to reconstruct for 

 ourselves, so to speak, the prairies of the state and to solve many 

 of the ecological questions which they call up. 



Some of the questions that have occurred to me are these r 



I. What were the conditions, climatic or of other nature, at 

 the close of the glacial epoch, which led to the invasion of prairie 

 plants from the west rather than forest plants from the southeast? 

 Certain climatic conditions are more favorable to the growth oi 

 prairie than of forest, notably a low winter rainfall, and it is note- 



I 



worthy that this type of rainfall is quite well developed in northern 

 Illinois, and thence west, but not in Indiana, or in southern Illinois. 

 Transeau's interesting map,^ comparing rainfall and evaporation, 

 is also pertinent here. Prairie plants complete their cycle of 

 development more rapidly than forest trees, and might enter the 

 territory sooner on that account. But if trees had migrated to 

 the north and west ever since the glacial period at a rate equal to 

 their present movement, the present state of Illinois would un- 

 doubtedly, by this time, have been covered entirely with forest. 

 If the American botanists will use more geological evidence in 

 their work, and if it can be used here as successfully as by the 

 German phytogeographers, Schulz for example, considerable light 

 will be throwh on this important point. The actual cause of the 



_ b. — ^ ^ H. ' ' "' "■■ H ■ ^ 



*Bot. Gaz. 31 : 73-108 ; 145-1S2. / i-jj. I901. 



f Gleason, Bull. 111. State Lab, Nat. Hist. 7 : 149-194. //. 8-2j. 1907. 



j: A more detailed paper on other inlai^d sand areas of the state is now in 

 preparation. 



g Harvey, Floral succession in the prairie-grass formation of southeastern South 

 Dakota. Bot. Gaz. 46: 81-108; 277-298. / 1-4. 190S. 



11 Pound and Clements, loc, cit. 



. The phytogeography of Nebraska. Lincoln, l^oo. 



Thornber, The prairie-grass formation in region I. Bot. Surv. Nebr. 5 : 29-143. 



1901 



^Forest centers of eastern America. Amer. Nat. 39: 875-889. f. 1-6, 1905, 



